Concrete mixes generally comprise a hydraulic cement, e.g. a Portland cement, a coarse aggregate in the form of gravel and a fine aggregate such as sand, which must be combined with water to form a pourable concrete mixture capable of setting up to considerable hardness. The cement, sand and gravel together form the so-called dry mix in spite of the fact that at least the aggregate (sand and/or gravel) may contain varying amounts of intrinsic moisture depending upon mode of preparation, method of storage and even weather conditions.
It is known to indirectly measure the consistency of a concrete mixture by electrical resistance methods and to empirically determine the water quantities which must be added to bring the ultimate concrete preparation or mixture to a desired or set point consistency.
In general, the consistency measurement is carried out on the concrete mix by determining the electrical resistance thereof and tubulated values of this consistency versus water additions for a given set point consistency, can be consulted to allow the preparation of concrete mixtures with the desired consistency. These tabulated values permit concrete mixes of various compositions and characteristics to be handled with comparative ease.
The water is generally supplied in increments (pulsed water feed) until the desired consistency value is reached by simply determining values of the water present for certain actual value consistencies and the set point (final) consistency for the respective mixes.
These techniques have the disadvantage that the time to completion of preparation of the mix is excessive, generally because of the time required for the incremental addition of water, and/or because the measurement by electrical resistance techniques of the actual-value consistency takes place in a consistency range at which the measurements are unreliable or nonreproducible, usually because of the high degree of dryness of the mix.
In fact, it has been found that varying concrete compositions in the presence of the small amount of moisture generally encountered when the actual value consistency is measured, require various measurement techniques or manipulation to obtain any significant results whatsoever. Relatively dry mixes, for example, necessitate determinations of specific electrical resistance while more moist mixes require determination of power consumption of the mixer drive in the measuring methods deemed optimal heretofore (see the aforementioned copending application). Obviously, earlier techniques are time-consuming and require complex and diverse apparatus and application to various concrete compositions.